Historically, hemp fibers have been used in the textile industry. However, recent breakthroughs in composite materials allowed renewable fibers, for example those from hemp, to replace glass fibers as strengtheners in composite materials. Therefore, the development of procedures to extract hemp fibers without damaging their integrity will facilitate their use in both the textile industry and in biocomposites. Such procedures would preferably be energy-efficient and would avoid the use of hazardous and/or non-biodegradable agents.
In the stem of fiber plants, such as hemp, flax and jute, a bark-like layer containing bast fibers surrounds a woody core or the stemwood. Decortication, either manually or mechanically, is a process that can divide the hemp stem into a hemp “bark” and a hemp “stem wood” fraction. The “stem wood” fraction can be utilized for chemical pulping. (Kortekaas 1998). “Bark” is used to describe all the outer tissues of the stem, including the bast fibers. The bast fibers or fiber bundles are surrounded by pectin or other gumming materials.
Plant fibers, are made of polysaccharides, mainly cellulose. This is different from animal fibers such as silks from silkworm and spiders, wool from sheep or other furry livestock, that are made of protein.
Isolation of plant fiber from the decorticated bark is required before any industrial application. Extraction primarily involves degumming, a removal of pectin from the fiber. Pectin is a polysaccharide which is a polymer of galacturonic acid. Pectin is not soluble in water or acid. However, it can be removed by strong alkaline solutions like caustic soda (concentrated sodium hydroxide).
General methods for isolation of clean fibers include dew retting, water retting, and chemical and enzymatic processes, with various modification. It involves the loosening or removal of the glue that holds the fibers together. The traditional methods are water- or dew-retting. In dew retting, stalks are allowed them to lie in the field after cutting. In some areas of the world, hemp is water-retted by placing bundles of stalks in ponds or streams. These two retting (limited rotting) methods depend on digestion of pectin by enzymes secreted by natural microbes. The water retting process has the disadvantage of polluting the waterway or streams. The dew-retting requires two to six weeks or more to complete, and very much affected by the weather with no guaranty of favorable conditions.
Enzyme retting involves the action of the enzyme pectinase with or without other enzymes like xylanase and/or cellulase. However, the practical application of such enzymes for isolation of hemp fiber remains in experimental stage.
Today the common industrial procedure is chemical retting which involves violent, hazardous chemicals like soda ash, caustic soda and oxalic acid, often at high temperature of 160° C. at pressures of several atmospheres.
Various retting processes are known in the art. Clarke et al. (Clarke 2002) describes a process of removing pectin or gummy materials from decorticated bast skin to yield individual fibers by placement of the bast skin (with or without soaking in an enzyme solution in a pretreatment process) into a closed gas-impermeable container such as plastic bag. The enzyme-producing microbes natural to the bast skin, will thrive on the initial nutrients released by the enzyme pretreatment and will finish the retting process in this closed environment. Clarke also describes an alternative pre-treatment process involving chemicals instead of enzymes, and this includes caustic soda, soda ash, sodium silicate, oxalic acid and ethylenediaminetetraacetic acid (EDTA).
Thus, there is a need for a milder and efficient process for isolating hemp fibers that involves environmentally-friendly and/or biodegradable agents. There is also a question of whether pectin being the only target for degumming. The removal of gumming matters other than the primary target, pectin, may offer the opportunity to yield finer and softer fibers of hemp.
Sung et al. (Sung 2007) taught that pre-treatment of the decorticated hemp bast skin with an aqueous solution containing di-sodium citrate, trisodium citrate or a mixture thereof having a pH of from about 6-13 at a temperature of about 90° C. or less, facilitates the subsequent extraction of fiber with the enzyme pectinase.
The hemp stem consists of both bast fiber (bark) and woody core (stemwood). The major components of these two parts are cellulose, hemicellulose, pectin and lignin (see Table 1) (Garcia-Jaldon 1998).
TABLE 1Chemical analysis of hemp partsBast fiber (%)Woody core (%)Cellulose5548Hemicellulose1612Pectin186Lignin428Wax + Fat11Ash42Protein23
In terms of chemical composition, the major differences between the bast fiber (bark) and the woody core (stemwood) are the amount of pectin (18% vs. 6%) and lignin (4% vs. 28%). The large amount of lignin in “stemwood” gives it rigidity. In the case of bast fiber (bark), the lack of lignin is compensated by pectin to glue the individual long fibers and fiber bundles together. Therefor, most research into the liberation of the long fiber from bark has been focused on hydrolysis of pectin, the major gumming component, through the application of the enzyme pectinase.
In comparison, the amount of protein is very small in the bast fiber (2% in bast fiber, Table 1). However, part of this seemingly unimportant protein is structural proteins like “extensin”, responsible for the protein matrix which contributes to the structural integrity of the plant itself. Application of protease to the bark may degrade the protein matrix, resulting in the release of non-fiber material or debris physically or chemically associated to the plant protein. As a result of such treatment, fiber may be released or separated.
Pokora et al. taught delignification of refiner mechanical wood pulps to facilitate biopulping, by use of protease at acidic pH (Pokora 1994). Pokora et al. taught that the proteases were used to delignify the wood by the wood protein “extensin”. “Extensin” is a cross-linked protein which is suspected of being bound to lignin and functions as a supporting skeleton on a cellular level. Since Pokora et al. is directed to the removal of lignin in mechanical wood pulps, it is not relevant to the isolation of the long fiber from “bark” which contains little lignin (Table 1).
Dorado et al. have taught the use of protease at neutral pH to remove lignin specifically from hemp “stemwood” through a pretreatment with protease (Dorado 2001). Similarly this is not relevant to the extraction of long fiber from bark.
Protease is commonly used in the purification of natural fibers of animal origins, like wool and silk. These fibers are also of protein origin, thus fundamentally different from the plant fibers which are of polysaccharides.
Protease has also been applied in the “bioscouring” of cotton fibers which has various layers of non-cellulosic materials including protein/nitrogenous substances. Cotton when harvested is “cotton boll”, which is a soft fluffy ball of already separated individual fibers. The removal of non-cellulosic materials from the surface of individual cotton fibers enhances wettability and ease of dyeing (Karapinar 2004). This is not for application in the separation or extraction of fiber from bark or bast skin of fiber plants. Bark or bast skin of fiber plants such as hemp or flax bark is quite different from cotton boll. Bark or bast skin is a sheet containing individual fibers all glued (or gummed) together into bundle, and then into a sheet. No individual fiber is visible at this stage. Although protein makes a small part of fiber plants, structural proteins like “extensin” interlock separated microfibrils (fine fibers) to reinforce the architecture. Other proteins may also be inserted to cross-link extensin, forming a network between fibers.
Instead of application of a single enzyme, purification of plant fibers may be done with commercial liquid enzyme mixtures produced directly through the culture of the fungus Aspergillus niger, including Novo SP249 (Akkawi 1990), or Pektopol PT-400 (Pektowin, Poland) (Sedelnik 2004; Sedelnik 2006). The decorticated fiber bark has to be treated with a bath containing this fungal enzyme mixture for as long as 24 to 36 hr. As expected, these natural enzyme mixtures obtained via culture of Aspergillus contain a wide-spectrum of its normal enzymes, including polygalacturonase, pectinase, cellulases, beta-glucanase, hemicellulases, xylanases, arabinase and protease in various amounts (Massiot 1989; Steinke 1991).
The abovementioned commercial enzyme mixtures (Novo SP249 and Pektopol), produced directly through the culture of fungus Aspergillus, are only suitable for application at acid pH with optimal pH range of 4-6 (Akkawi 1990; Sedelnik 2006; Steinke 1991). Towards neutral pH, the Aspergillus enzymes lose activity rapidly.
As to the effect of long treatment time on plant fiber at acidic (low) pH, Jaskowski (Jaskowski 1984) teaches that acidic treatment solutions at pH below 4.5 can promote acidic hydrolysis of plant fiber, which is primarily cellulose, and that significant degradation of decorticated bast fiber happens if the fiber remains in such treatment solutions for longer than 1 hr. Since treatment with fungal enzyme mixtures as described above lasts 24 hr or longer, damage to the integrity of the purified fiber is a matter of concern.